refactor(google-health): read and write backfill progress through one schema

The backfillProgress column was read by four hand-rolled walkers over
untyped records and written inline in the sync core. One module now owns
the column: a Zod schema reads it, lenient per entry so a malformed cursor
or end is dropped on its own, and one function writes it. The sync core
keeps only the bookkeeping of a running backfill.

src/app/api/integrations/status/route.ts

@@ -55,7 +55,7 @@ import { loadConnectionHealth } from "@/lib/integrations/connection-health-load"
 import {
   googleHealthBackfillEndedList,
   type GoogleHealthBackfillEndedEntry,
-} from "@/lib/google-health/sync-core";
+} from "@/lib/google-health/backfill-progress";
 import type { ConnectionHealth } from "@/lib/integrations/connection-health";
 
 export const dynamic = "force-dynamic";